Hsp90 is a protein chaperone that utilizes the hydrolysis of ATP to assist in the folding of early nascent forms of proteins to their mature, correctly-folded forms. Once the protein has been correctly folded, Hsp90 is released and thus, it functions as a true protein “catalyst.” Hsp90 has also been recognized as an attractive anticancer target in that this chaperone assists in the folding of many oncogenic proteins including ErbB2, Raf-1, mutant p53, estrogen and steroid receptors. Thus, by inhibiting Hsp90, a large number of downstream oncogenic proteins can be disrupted, thereby attacking the neoplastic process at a number of points.
The first Hsp90 inhibitor used clinically was geldanamycin. Geldanamycin is a benzoquinone ansamycin polyketide isolated from Streptomyces geldanus. Although originally discovered by screening microbial extracts for antibacterial and antiviral activity, geldanamycin was later found to be cytotoxic to tumor cells in vitro and to reverse the neoplastic morphology of cells transformed by the Rous sarcoma virus.
Unfortunately, the administration of geldanamycin produced unacceptable hepatotoxicity, which led to its withdrawal from Phase I clinical trials. The toxicity of these compounds is believed to be a result, at least in part, of glutathione depletion. Second generation geldanamycin derivatives were developed including 17-demethoxy-17-(2-propenylamino)-geldanamycin (17AAG; also known as 17-allylaminogeldanamycin) and 17-demethoxy-17-[[2-(dimethylamino)ethyl]amino]-geldanamycin (17-DMAG). These molecules reduce liver toxicity and have shown success in Phase I and Phase II clinical trials.
While there has been a great deal of research interest in the benzoquinone ansamycins, particularly geldanamycin and 17-AAG, there remains a need for effective derivatives of these compounds having higher activity without the significant risk of toxicity of the parent geldanamycin compound.